1. Field of Invention
The present invention relates generally to apparatus and methods for use in the precision cleaning of substrate surfaces. More particularly, the present invention relates to apparatus and methods for removing trace contaminants from delicate surfaces and precision hardware such as that found in high sensitivity optical, electronic and precision mechanical equipment.
2. Description of Related Art
It is essential that delicate and precision surfaces of optical-mechanical-electronic equipment be contaminant-free. For example, infrared optical sensor systems use thin film optical coatings to enhance in-band transmittance and reject out-of-band radiation. Even trace amounts of contaminations degrade spectral performance by scattering, absorbing, or reflecting incident radiation.
Particulate and molecular contaminants present on optical surfaces can cause problems by changing emittance characteristics thereby increasing thermal noise in optical devices. Dust and debris contamination present on a detector surface may also generate clutter in the appearance of false targets. Further, the presence of molecular contaminants on precision optical equipment surfaces, such as lenses, windows and optical filters, results in the absorption and scatter of incident energy with the resultant decrease in system quality.
In precision computer and gyroscope equipment, particulate contamination in a moving disk drive or in a spinning gyroscope stator may damage or disable such a system beyond repair. Technologies used in manufacturing integrated circuits also require that substrates be free of particulates and molecular contaminants.
Precision cleaning apparatus and methods are not limited in use to high technology applications. For example, the presence of particulate and molecular contaminants on such substrates as automobile body panels impairs the integrity of the coatings applied thereon and may result in peeling or bubbling of the coated surface.
A wide variety of cleaning solvents and solutions, such as chlorofluorocarbons and ketones, have been utilized to clean delicate surfaces. However, the majority of these cleaning solutions and solvents are environmentally hazardous. In response to these environmental concerns, a number of environmentally safe cleaning mediums had been proposed, one of which is carbon dioxide.
More particularly, a variety of investigators have reported utilizing a stream or Jet spray of carbon dioxide, also known as "dry ice snow", to remove molecular and particular contaminants from a variety of surfaces including silicon wafers, telescope mirrors and thin film optical coatings.
To form a carbon dioxide Jet spray, liquid carbon dioxide from a cylinder is expanded through a device which has an orifice and a nozzle. Rapid expansion of the carbon dioxide liquid causes the formation of fine particles of solid CO.sub.2 which become entrained in the carbon dioxide gas. The size of the orifice and the nozzle configuration are adjusted to obtain the desired CO.sub.2 snow. With this configuration, a jet of high velocity snowflakes is produced and is directed at the surface to be cleaned. The term "snow" is used herein to mean a stream of gas entraining solid particles of such gas.
The formation and use of carbon dioxide snow is disclosed in the following articles and patent, the contents of which are hereby incorporated by reference: S. A. Hoening, "Cleaning Surfaces with Dry Ice", Compressed Air Magazine, August 1986; R. V. Peterson, C. W. Bowers, "Contamination Removal by CO.sub.2 jet spray," SPIE, Vol. 1329, Optical System Contamination Effects, Measurements, Control II, 1990; L. Layden, D. Wadlow, "High Velocity Carbon Dioxide Snow For Cleaning Vacuum System Surfaces," Journal of Vacuum Science and Technology, A8 (5) September/October 1990; R. Sherman, W. H. Witlock, "The Removal of Hydrocarbons and Silicon Grease Stains from Silicon Wafers", Journal of Vacuum Science Technology, B8 (3) May/June 1990; R. Zito, "Cleaning Large Objects with CO.sub.2 Snow", SPIE Vol. 1236, Advance Technology Optical Telescopes IV, 1990; U.S. Pat. No. 4,806,171 for "Apparatus and Method for Removing Minute Particles from a Substrate" issued to Whitlock et al.
The use of carbon dioxide snow to clean delicate surfaces has shown promise. However, as with any new technology, there is a continual need to further develop and improve the state of the art.